Editor: Add loading state to the 'PostAuthorCombobox' component

[adamsilverstein]

What?

Add a loading indicator to the author dropdown. Fix a bug where it was impossible to tell a search was occuring, leating to poor UX especially over slower connections.

Follow up to #68990

Post author part of #68927

Testing instructions

1. Create a large number of editors (see below for command).
2. Load a page in the editor
3. Open DevTools and throttle network connections to a 3G speed to slow down the rest API responses
4. Click the author to edit, then try typing in part of a user name

Expected results:

• A spinner is shown while the search callback executes
• the "No items found' string only shows when the search completes (and no items are found)

Helpful wp-cli commands for testing:

Generate many users to test searching: wp user generate --role=editor --count=100
Delete test users: wp user delete $(wp user list --role=editor --field=ID) --reassign=2

Screencasts

Before changes (trunk)

before2.mp4

Add spinner

(note message "No items found" shows during search callback)

spinner-only.mp4

Final PR

(including removal of "No items found" during load)

after.mp4

[github-actions]

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[Mamaduka]

Question (non-blocking): Should we track the loading state for every query or just for the search?

[adamsilverstein]

Good question, my assumption was that search was the only async query, are there others?

If we did expand this to all queries, what would that look like? Does the isResolving call change?

[Mamaduka]

The initial query is also async, but I guess the loading state could also be a helpful hint.

Looks good to ship as it is ✅

[Mamaduka]

This should be good to merge after #68990.

Left one question, but it's not a blocker.